The third generation partnership project (3GPP) has defined a concept to enable mobile operators to restrict access to a cellular base station (or a group of cellular base stations) to a limited group of subscribers. This group of subscribers is called a Closed Subscriber Group (CSG). The notion of CSGs is realized by assigning a unique Closed Subscriber Group ID (CSG ID) to the cell or the group of cells that access shall be limited for and by broadcasting the CSG ID as well as a CSG indicator, which declares the cell to be a closed cell, on the cell's broadcast channel. Furthermore, each subscriber's subscription contains a list of CSG IDs, which denote the CSG cells that this subscriber is allowed to access (the so-called Allowed CSG list).
Based on this information, the mobile radio access network (hereinafter also called system) performs access control: when a subscriber (or a subscriber's terminal) attempts to access the network via a CSG cell, the system checks whether the cell's CSG ID is contained in the subscriber's Allowed CSG list. If so, the access is allowed, otherwise the access is denied.
In addition to such radio cells having restricted access (i.e., which are closed), hereinafter also called CSG cells, 3GPP has defined the notion of hybrid cells. Hybrid cells are defined as a combination of CSG cells and open cells (i.e., non-CSG cells). In other words, hybrid radio cells comprise a CSG part and an open part. The key idea is that any subscriber of the mobile radio access network is allowed to access the open part of a hybrid cell but that only members of the CSG part of the hybrid cell have access to the CSG part of the hybrid cell, thus, for example, receiving a better service in terms of quality of service, bandwidth or the like.
The CSG concept and the concept of hybrid cells is explained in the following technical specifications: 3GPP TS 23.002, “3rd Generation Partnership Project; Technical Specification Group Services and Systems Aspects; Network architecture (Release 9)”, 3GPP TS 23.401, “3rd Generation Partnership Project; Technical Specification Group Services and Systems Aspects; Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access (Release 9)”, 3GPP TS 23.060, “3rd Generation Partnership Project; Technical Specification Group Services and Systems Aspects; General Packet Radio Service (GPRS); Service description; Stage 2 (Release 9)”. With regard to details of the CSG concept and the concept of hybrid cells, these three documents are incorporated by reference herein.
It is desirable to perform differentiated charging for CSG cells, hybrid cells and non-CSG cells (e.g. non-CSG macro network cells). An example for differentiated charging is to charge lower rates when a subscriber is accessing the network via a CSG cell (which might for example be a CSG-concept based femto cell or home base station that is installed in the subscriber's home) in comparison to the rates when the subscriber accesses the network via a non-CSG cell (e.g., a non-CSG macro cell).
However, there have not been any efficient solutions to enable this type of differentiated charging for CSG cells. One possible solution would be to report any change of cells for each subscriber to the network element which performs charging, hereinafter also called the charging unit (e.g., the GPRS Gateway Support Node, GGSN, or the Packet Data Network Gateway, PDN-GW in 3GPP systems). This would mean that each time a subscriber performs a handover to another cell or performs a signalling interaction in another cell, the ID of this cell is reported to the network element responsible for charging (the charging unit). Based on this cell ID, the charging network element can derive (using an additional database) whether the reported cell ID belongs to a CSG cell or hybrid cell and whether the subscriber is a member in this CSG—and can leverage this information for differentiated charging. However, the disadvantage of such an approach would be massive signalling overhead because (a) the change of cells needs to be reported for all cells in a network irrespective of whether the cell a subscriber is currently accessing is a CSG cell, a hybrid cell or a non-CSG cell, and (b) especially for highly mobile transceivers, cell changes happen very frequently.